The present invention relates to a discharge lamp especially suited for forward lighting applications of a vehicle such as an automobile, truck, bus, van or tractor. More particularly, the discharge lamp is a xenon-metal halide lamp for an vehicle headlamp having instant light capability, a relatively long life, and a relatively high efficiency.
Automotive designers are interested in lowering the hood lines of cars in order to improve their appearance and also their aerodynamic performance. As discussed in the cross-referenced U.S. Pat. No. 4,868,458, the amount that the hood lines may be lowered is limited by the dimensions of the automotive headlamp, which, in turn, is limited by the dimensions of the light source itself which is typically comprised of a tungsten filament.
As disclosed in U.S Pat. No. 4,868,458, a xenon discharge light source having dimensions which are substantially reduced relative to a tungsten light source allows for the reduction of the overall size of the reflector of the automotive headlamp so that the hood lines of the automobile may be substantially reduced by the automotive designers. In addition, the disclosed xenon discharge light source has an instant start capability similar to a tungsten filament and therefore is particularly suited for automotive applications.
The xenon light source while serving its desired functions suffers some disadvantage in that its efficiency is less than that of other discharge lamp types such as a metal halide lamp. This disadvantage is partly due because the operating voltage of the xenon lamp, finding use in automotive applications is relatively low such as 15 volts. This causes a large fraction of the energy consumed by such a xenon lamp to be dissipated by the electrodes of the xenon lamp rather than contributing to the light output. A further reason for the lower efficiency is that the xenon spectrum contains a relatively large amount of infrared energy which serves no useful purpose for automotive applications and is also detrimental to the plastic housing of the automobile headlamp.
It is desired that a discharge lamp such as a metal halide lamp be provided so as to serve the needs of the automotive headlamp. It is further desired, that the metal halide lamp provide for substantially instant light output such as that of a xenon lamp or tungsten incandescent light source. Still further, in addition to the metal halide lamp serving the needs of automobile, it is desired that the metal halide lamp find lighting applications in the home, office and other commercial and industrial usages.
Accordingly, it is an object of the present invention to provide a metal halide discharge light source for lighting applications and which is particularly suited to serve the needs of the automobile by allowing for substantially instant light.
It is a further object of the present invention to provide a metal halide discharge lamp having relatively small dimensions so as to allow for reduction in its related reflector of the headlamp, which, in turn allows for a reduction in the hood lines desired for aerodynamically styled automobiles.